The invention relates to a method of recovering hydrocarbons from an underground formation via a well which is at least partly surrounded at the level of the formation by sand grains.
Within the meaning of the claims and the specification the expression "sand grains" is to be understood to relate to grains substantially consisting of silica. The sand grains either consist of grains that are originally present in the formation, or of grains that have been introduced in or around the well for filling up voids around the well, for forming a gravel pack, or for any other reason.
The inflow into the well of sand grains that are entrained by the flow of fluid passing out of the formation into the well should be prevented. Such inflow of sand grains results in sanding-up of the well which requires costly cleaning operations when the production rate of the well decreases. Further, the tubing in the well as well as the surface equipment will be damaged by the erosive action of those sand grains that are passed to the surface by the fluid flow.
Inflow of sand grains into a well will occur when the bond between the formation sand grains is insufficiently strong to withstand the forces enacted thereon by the flow of fluid passing through the pore space of the formation into the well. One manner to solve this sand problem is to place a gravel pack in the well, such pack consisting of an unconsolidated volume of sand grains of predetermined size, which grains are kept in place by a screen pipe. The screen pipe is a short string of casing protecting the face of the producing formation and preventing, by means of its screening property, the formation sand grains from flowing into the well.
Failure of a gravel pack occurs when hot aqueous fluids pass through the pack over extremely long periods. The grains are partly dissolved in the hot fluid and the size reduction of the grains that accompanies such dissolution allows the grains from passing through the openings of the screen pipe and entering the well.
Further, a sand problem will be met in a well when passing hot aqueous fluid through an adjacent siliceous earth formation wherein the sand grains are interconnected by a silicate cementing material. Whereas no sand problem will exist in such well when low temperature fluids are passed therethrough, it will be found, however, that hot aqueous fluids dissolve the silicate cementing material thereby weakening the bonds between the grains until the strength of the bonds is insufficient to withstand the forces enacted on the grains by the fluid flow passing from the formation into the well. The sand grains are then sheared off and enter the well.
The majority of the consolidation processes that are suitable in formations wherein relatively cold fluids pass through the wells and the formation surrounding the wells, have been found to fail when the recovery process carried out in the formation is replaced by a process that includes the use of hot aqueous fluid. There are a limited number of processes, however, that render the formation resistant against the attack of hot fluids, but these processes are too costly for actual application. One such process is described in U.S. Pat. No. 3,393,737. A metal plating compound in the liquid phase is injected into the formation and deposits an uninterrupted metal layer over the surface of sand grains contacted by the compound. Although this metal layer forms an effective protection against degradation by hot aqueous fluid passing along the sand grains and the natural bond--if present--between the grains, no wide-spread application of this process has taken place since the process is chemically complicated and thus expensive as highly qualified chemists should be in charge of the operation.
U.S. Pat. No. 4,323,124 describes a method for inhibiting gravel pack and formation sand stone dissolution during water or steam injection, by coating the surface of the sand and the gravel with a fatty film of lecithin. A drawback is that the lecithin requires the presence of a dispersant to disperse it in the water phase of the treating liquid.
In U.S. Pat. No. 3,438,443 it is suggested to solve the dissolution problem by dissolving silica in the liquid phase of the hot fluid to be injected. By using a saturated solution of silica, the sand formation is protected from being leached by the hot fluid. Even sand grains that have previously been consolidated by an emulsion of water glass and kerosene followed by an aqueous solution of calcium chloride were found to be insoluble in the hot aqueous drive fluids oversaturated by silica. The tests wherein an oversaturated silica solution was used as a hot aqueous fluid indicated that no degradation of the consolidated sand took place, contrary to the use of solutions of sodium aluminate that were found to degrade the consolidated sand almost as rapidly as fresh water.
A drawback, however, of the use of oversaturated silicate solutions is that the amount of silica that is deposited on the sand grains when the solution cools down in the formation, will decrease the permeability of the formation, which requires continuously increasing pressures to maintain a constant injection rate of the hot aqueous fluid into the formation. Such increased injection pressures may even lead to fracturing of the formation, which results in by-passing of large formation parts by the injection fluid.
U.S. Pat. No. 3,603,399 discloses that in water-sensitive reservoir formations the adverse effects of clay swelling and clay migrating can be reduced by contacting the formation with an aqueous hydroxy-aluminum solution having a ratio of hydroxyl groups to aluminum atoms in the range of 1.5 to 2.7. U.S. Pat. No. 3,756,315 discloses that a basic aqueous solution of an alkali metal aluminate and a pH reducing reactant for subsequently reducing the pH to one at which a hydrated aluminum oxide is precipitated can be used to plug, consolidate, or coat, a reservoir formation. Neither of those problems or solutions are indicative of what might be done regarding the problems due to the dissolving of siliceous sands or cements.